Geothermal pipe collector

ABSTRACT

A geothermal pipe collector is provided. The geothermal pipe collector is made from a polymer composition comprising: more than 50 wt % polyethylene, 0.1 wt %-35 wt % talc and 0.5 wt %-10 wt % carbon black.

TECHNICAL FIELD

The present application relates to pipe collectors for geothermal heatexchange and polymer compositions for improving the properties ofgeothermal pipe collectors.

BACKGROUND

Geothermal energy is energy stored as heat in the ground. This energymay originate from the hot core of the earth or may be heat generated bythe earth surface being exposed to infrared radiation from the sun. Mostgeothermal installations today use the second category of geothermalenergy, i.e. solar energy stored as heat in e.g. water, ground orbedrock.

In a geothermal energy system using circulating fluid, the heat isextracted from the ground (e.g. water or bedrock) using a geothermalpipe collector. In the geothermal pipe collector, the fluid, known asheat transfer medium or heat transfer liquid is circulated such thatfluid heated by the geothermal energy is extracted in one end of thegeothermal pipe collector, the cooled fluid is then returned in theother end of the geothermal pipe collector such that a closed system iscreated.

Examples of geothermal energy systems are ground surface heat systems,sea heat systems and borehole heat systems. In ground surface heatsystems, a several hundred meter long geothermal pipe collector isburied in the ground at a frostproof depth. In a sea heat system, a

In a borehole heat system, a geothermal pipe collector having two fluidconduits is placed in the bore hole, such that the fluid can be conveyedinto the bore hole in a first fluid conduit and conveyed from the borehole in a second fluid conduit. The geothermal pipe collectors for usein borehole heat systems could be so called U-pipe collectors. A U-pipecollector comprises a separate, closed pipe which is bent such that itforms a U-shape, such that the direction of the fluid conveyed isaltered in the bottom of the borehole. An alternative geothermal pipecollector is the so called double U-pipe collector, which comprises twopipes for conveyance of the heat transfer liquid down into the drilledhole which branches off into two pipes that transports the fluid backfrom the drilled hole and further to a heat pump.

Yet another type of collector used in borehole heat systems is the socalled coaxial collector. In the coaxial collector, an inner pipe isarranged in an outer pipe. The pipes are welded together, such that asingle unit is formed, and subsequently installed in a drilled hole. Thefluid is conveyed down into the borehole in the outer pipe and thusabsorbs heat from the borehole. When the fluid reaches the bottom of theborehole it is conveyed up again through the inner pipe. It is desirableto avoid long surface area contact between heated fluid and the cooledfluid, which is why the outer pipe is provided with a largercross-sectional area, such that a faster flow is achieved in the innerpipe.

The heat or cooling collected by means of any of the collectorsdescribed above is used to vaporize or condensate a cooling agent of aheat pump in the system, and thereby heat or cooling is extracted fromthe circulating fluid.

Due to its resistance against degradation by the environment of theground, Polyethylene (PE) is a material widely used for themanufacturing of geothermal pipe collectors. However, PE is generallyviewed as a thermal insulator with low thermal conductivity, which is adrawback when the material is used in heat exchange applications. Forthe purpose of decreasing the thermal resistance in the pipe collector,the wall thickness of the pipe collector may be decreased. However,decreasing the wall thickness affects the mechanical properties of thepipe, which may be a disadvantage for fulfilling the requirements ofpipe standards and for the handling of the pipe. In EP 2195586 to M.Ojala et al., it is described how the thermal conductivity of ageothermal pipe collector can be increased by creating a turbulent flowof the fluid inside the pipe collector. However, creating a turbulentflow involves the creation of a groove or recess in the wall of thegeothermal pipe collector, which decreases the thickness of the wall ofthe geothermal pipe collector, again affecting some of the mechanicalproperties of the pipe.

To be able to reduce the length of the pipe used in ground surface heatsystems, and reduce the depth of the borehole in borehole heat systems,it would be advantageous to have a design of a geothermal pipe collectorwith reduced thermal resistance and maintained mechanical properties.

SUMMARY

A geothermal pipe collector is provided. The geothermal pipe collectoris made from a polymer composition comprising: more than 50 wt %Polyethylene (PE), 0.1 wt %-35 wt % talc and 0.5 wt %-10 wt % Carbonblack (CB). The addition of CB protects the geothermal pipe collectoragainst the natural environment but reduces some of the mechanicalproperties of the polymer composition. The addition of talc increasesthe performance of a thermal conductivity thus increasing the heatexchanging capabilities of the geothermal pipe collector making itpossible to have the same heat exchanging capabilities with a shorterpipe collector. The addition of talc also increases the relevantmechanical properties of the pipe collector, which makes it possible tohave thinner walls, which further decreases the thermal resistance ofthe pipe collector and thus increases the heat exchange.

According to one embodiment, the geothermal pipe collector is made froma polymer composition comprising 0.1 wt %-3 wt % talc.

According to one embodiment, the geothermal pipe collector is made froma polymer composition comprising 8 wt %-35 wt % talc. By adding morethan 8% talc, the density of the pipe collector is increased such thatthe pipe collector gets a higher density than water and thus sinks in aborehole or in sea water.

According to one embodiment, the geothermal pipe collector is made froma polymer composition comprising 8 wt %-15 wt % talc. This embodimenthas a higher density than water and is still flexible enough to easilybe coiled.

According to one embodiment, the geothermal pipe collector is made froma polymer composition comprising 8 wt %-12 wt % talc. In the interval 8wt %-12 wt % talc, the polymer composition have some mechanicalproperties being substantially the same as PE without the addition ofCB.

According to one embodiment, the geothermal pipe collector is made froma polymer composition comprising 0.5 wt %-5 wt % CB or 1.5 wt %-3 wt %CB. Both compositions provide sufficient protection against the naturalenvironment, but a polymer compositions comprising 3 wt %-5 wt % CB havea higher thermal stability.

In any of the embodiments herein, the talc added to the polymercomposition may be talc in which the average aspect ratio is above 1.2.Talc with a high aspect ratio further increases the thermal conductivityof the polymer composition.

According to one embodiment, the inner surface of the geothermal pipecollector comprises recesses or protrusions for increasing theturbulence of a medium flowing in the pipe collector and thus the heatexchange in the pipe collector. The recesses or protrusions may extendhelically on the inner surface of the pipe collector, in relation to thelength axis of the pipe collector, such that a turbulent flow is createdin the direction of the length axis of the pipe collector. In oneembodiment, the helically extending recesses or protrusions alterdirection at least at some portion along the length axis of the pipecollector, such that the direction of the turbulent flow is alteredalong the length axis of the pipe collector, which introduces furtherturbulence of the fluid.

The recesses or protrusions on the inner surface of the pipe collectormay extend continuously on the inner surface of the pipe collector, suchthat the pipe collector can be manufactured by means of continuousextrusion.

Please note that any of the polymer compositions or any combinations ofadditives mentioned herein could be used with any type of geothermalpipe collector without departing from the basic idea of the invention.

BRIEF DESCRIPTION OF DRAWINGS

The invention is now described, by way of example, with reference to theaccompanying drawing, in which:

FIG. 1 shows a geothermal U-pipe collector in a borehole,

FIG. 2 is a table of polymer compositions on which experiments have beenconducted,

FIG. 3 is a graph showing the density of a polymer composition as afunction of the filler contents,

FIG. 4 is a table showing the tensile strength of different polymercompositions,

FIG. 5 is graph showing tensile strength of polymer compositions as afunction of the filler content in the compositions,

FIG. 6a is a graph showing impact resistance as a function of fillercontent in polymer compositions, notched and edgewise,

FIG. 6b is a graph showing impact resistance as a function of fillercontent in polymer compositions, un-notched and flatwise,

FIG. 7 is a graph showing elongation and impact resistance as a functionof the filler content in the polymer composition,

FIG. 8 is a graph of thermal conductivity and thermal diffusivity ofpolymer compositions as a function of the filler content in the polymercomposition,

FIG. 9 is a graph of the volumetric heat capacity and specific heat ofpolymer compositions as a function of the filler content in the polymercomposition,

FIG. 10a is a graph showing the thermal resistance of a pipe collectoras a function of the thermal conductivity of the polymer composition ofwhich the pipe is manufactured,

FIG. 10b is a graph showing the thermal resistance of a pipe collectoras a function of the thermal conductivity of the polymer composition ofwhich the pipe is manufactured, for different pipe dimensions,

FIG. 11 is a table showing the weight loss of different polymercompositions,

FIG. 12 is a graph showing the weight of a polymer composition as afunction of temperatures to which the polymer composition is exposed,

FIG. 13 is a graph of the particle size of a polymer composition byshowing the percent of the volume which is made up of particles having aspecific diameter,

FIG. 14a shows an embodiment of a geothermal pipe collector in which thepipe collector comprises helically extending recesses,

FIG. 14b shows an embodiment of a geothermal pipe collector, in whichthe pipe collector comprises helically extending recesses which alterdirection at least at some portion along the length axis of the pipecollector.

DETAILED DESCRIPTION

In the following, a detailed description of embodiments of the inventionwill be given with reference to the accompanying drawings. It will beappreciated that the drawings are for illustration only and are not inany way restricting the scope of the invention. Thus, any references todirections, such as “up” or “down”, are only referring to the directionsshown in the figures.

FIG. 1 shows the principle for a conventional U-pipe collector.According to this principle, a continuous, sealed Polyethylene (PE) pipe1 is arranged in a drilled borehole 2. The pipe 1 is preferably made asa single continuous pipe extruded in a plastic extruder. The pipe 1forms a U-shaped bend 3 in the end towards the bottom 4 of the borehole,such that the fluid conveyed into the borehole 2 is conveyed up againafter reaching the bottom of the borehole 2. This particular system isknown as a “U-pipe collector” as the bend forms a U-shape. FIG. 1 onlyshows the principle, the U-shape of the pipe collector may be a separatepart to which a first and second portion 1′, 1″ of the pipe is welded.The upper part 5 of the collector system is usually terminated in amanhole at ground level 6, from where the collector pipes 1, 1′, 1″ areconnected to a heat pump (not shown). During assembly, the pipe is fedinto the borehole 2 manually or by means of a pipe feeder. The pipefeeder typically comprises pulleys feeding the pipe, and the feeder thusbends the pipe while pressing it into the borehole. The feeding thusplaces substantial strain on the pipe and the pipe thus needs to be bothflexible and capable of handling considerable strain.

To withstand the ground environment and the strain induced by thefeeder, the geothermal pipe collector is made from a polymer compositionbased on PE. A polymer composition is to be understood as any compoundedmaterial comprising at least one polymer material in some quantity, anda filler is to be understood as a material in the polymer compositionother than the main polymer (herein PE). The fillers described hereinare talc and CB, and talc is the filler in instances in the graphs wherethe filler is not specified.

For the purpose of increasing the outdoor stability and in particularthe UV resistance of the PE, Carbon black (CB) is added to the polymercomposition. CB is a form of amorphous carbon produced by incompletecombustion of petroleum products. CB is an economic additive whicheffectively increases the UV resistance of PE even at low concentrations(such as between 0.5% and 3%). CB can also be used to increase thethermal stability of PE, which is important when the pipe collectors areused in high temperature applications. The drawback however, is that CBreduces some of the mechanical properties of PE, making the materialhard and brittle.

Talc is a mineral composed of hydrated magnesium silicate arranged inthree disc shaped layers. In the middle, there is a layer ofmagnesium-oxygen/hydroxyl octahedra, while the two outer layers arecomposed of silicon-oxygen tetrahedra. These layers are kept togetheronly by van der Waals' forces, and the layers have the ability to slipover each other easily, which makes talc the softest known mineral,measured as 1 on the Mohs hardness scale. The talc's uniquecharacteristics such as softness, chemical inertness, slipping, oil andgrease absorption, whiteness, availability, and its rather low price,makes it a promising material to be used as a filler.

For the purpose of decreasing the thermal resistance of the geothermalpipe collector and enhancing the mechanical properties of the polymercomposition, talc is added to the polymer composition. The talc filledpolymer composition decreases the thermal resistance of the geothermalpipe collector by increasing the thermal conductivity of the materialand enabling the reduction of the wall thickness of the geothermal pipecollector, thereby increasing the heat exchange.

In one embodiment, the geothermal pipe collector is made from a PE basedpolymer composition comprising: 0.1 wt %-35 wt % talc and 0.5 wt %-10 wt% CB (and the remaining part PE). The CB protects the geothermal pipecollector against the elements of nature, and to have a sufficientprotection against UV-radiation, at least 0.5% should be included in thecomposition. The addition of CB also improves the heat stability of thepolymer composition. However, as previously mentioned, the CB negativelyaffects some of the mechanical properties of PE, in particular theimpact resistance. The added talc increases the thermal conductivity ofthe polymer composition as well as the modulus of elasticity and thetensile strength, and increases the density of the polymer composition.Apart from that, the addition of talc reduces equipment wear duringprocessing, decreases shrinkage, and improves the product machinability.Also, the addition of talc reduces the specific heat capacity of thecomposition, which makes it possible to increase production speed.

In some applications, additional additives may be needed in the polymercompositions for further increasing the thermal stability or mechanicalproperties of the PE. Such additives may include Kaolin clay, silica andcalcium carbonate, or dye for obtaining a pipe collector of a specificcolor.

In one embodiment the geothermal pipe collector is made from a polymercomposition comprising 0.1 wt %-3 wt % talc. The conductivity of thepolymer composition may be increased at small levels of talc, as thecrystallinity of the polymer composition is increased.

In one embodiment, the geothermal pipe collector is made from a polymercomposition comprising 8 wt %-35 wt % talc. Above 8 wt % talc, thepolymer composition has substantially the same impact resistance as PEwithout the addition of CB (denoted as PEn in the tables and diagramsherein). Above 8% of talc, the polymer composition will have a densityof more than 1 (i.e. higher than water), which makes the geothermal pipecollector sink to the bottom of the borehole, which is a clear advantageas no active feeding of the pipe collector is needed. Also, the pipecollector having a density of above 1 could be used in sea heatapplications without the need to anchor the pipe collector to the seabed.

Adding more than 35 wt % of talc in the polymer composition will createa polymer composition having an elasticity modulus too high for normalextruders, and even if the material could be extruded with highpressure, the finished pipe collector would be difficult to coil or feedinto the borehole without the risk of breakage, which will make thematerial very hard to transport and handle. Also, the risk that thematerial is not properly compounded increases as the amount of talc isincreased, creating a risk that large areas of poorly mixed polymercomposition will be created, which increases the risk of breakage.

According to one embodiment the polymer composition comprises 8 wt %-15wt % talc. In this interval, the polymer composition (with 0.5 wt %-3 wt% CB) has density above 1 and substantially the same impact resistanceas PE without CB. As can be seen in for example the graph of FIG. 7, theimpact resistance of the polymer composition is reduced considerablyabove 15 wt % talc, which makes the polymer composition less suitablefor some geothermal pipe collector applications. In applicationsbenefitting from the wall of the geothermal pipe collector being thin,the polymer composition could comprise 8 wt %-12 wt % talc, which is theinterval (depending on the amount of CB in the composition) in which theimpact strength is highest at the same time as the density is above 1,making the geothermal pipe collector sink in the borehole or when placedin the sea. When the polymer composition comprises approximately 2.5 wt% CB, the impact resistance, as measured by the Charpy impact test, isreduced by almost 30% (as can be seen in the graph of FIG. 6). Forregaining the impact strength of PE without the CB (PEn), between 8 wt%-12 wt % of talc needs to be added to the composition, which, as statedabove, also increases the density of the material such that the materialwill have a density higher than water.

For the purpose of optimizing the talc's effect on the conductivity ofthe polymer composition, the talc could have a high aspect ratio. Theplate-like shape of high aspect ratio talc adds to the enhancement ofthe impact strength and the thermo-physical characteristics However,when the talc is compounded into the polymer composition, the particlesare fractured or altered in shape, which decreases the aspect ratio ofthe talc particles in the polymer composition. In one embodiments, thepolymer composition including high aspect talc means that the aspectratio of the talc particles on average is above 1.2. In otherembodiments, the polymer composition including high aspect talc couldmean that the aspect ratio of the talc particles on average is above1.5, and in yet another embodiments, the polymer composition includinghigh aspect talc could mean that the aspect ratio of the talc particleson average is above 2.

As talc has a plate-like shape with a high aspect ratio and the layersof talc can easily slip over each other during the processing, thepolymer can easily fill the spaces between the particles this wouldhappen if there is sufficient shear stress during the pipe processing orcompounding. Particulates can therefore be oriented in the flowdirection parallel to the axis of the pipe surely particle orientationdepends on the nature of the flow field, but it is quite clear from SEMimages that particles are well dispersed and oriented with the injectionmoulding direction. This unique organization allows forming kind of heatchannels as result heat can be transferred through, although theparticulate did touch completely together to make the true heat channel.

The addition of CB in the polymer composition increases UV resistance ofthe polymer and increases the thermal stability. However, when above 5wt % CB has been added to the polymer composition the mechanicalproperties have decreased substantially, which makes the material verybrittle and increases the risk that the material will break duringhandling, and makes the material hard to transport as a coil. Thepolymer composition could therefore, according to one embodiment,comprise 0.5 wt %-5 wt % CB (up to 5 wt % for increased thermalstability), and for the purpose of providing sufficient UV-protectioncomprise 1.5 wt %-3 wt % CB.

Depending on the amount of additives in the polymer composition, inparticular CB and talc, the amount of PE in the finalized compositionvaries. However, the polymer composition preferably comprises more than75 wt % PE.

Although a single pipe collector in the shape of a U-pipe collector isdescribed with reference to the figures, the polymer compositionsdescribed could just as well be used in other types of collectors, suchas coaxial collectors or double U-pipe collectors. Any of the geothermalpipe collectors may be adapted for borehole heat systems, ground surfaceheat systems or sea heat systems or any geothermal cooling system.

Experiments Performed on Polymer Compositions

In the following, tests performed on material compositions will bepresented. The specific compositions tested are to be seen asexemplifying embodiments supporting the inventive benefits of thecompositions presented herein, and are not be seen as restricting thescope of the present invention.

In the following experimental embodiments, High-Density Polyethylene(HDPE) was used as a matrix material, which was supplied by UnipetrolRPA, Czech-Republic. The HDPE has a melt flow rate (MFI) of 0.4 g/10min, a Vicat softening temperature of 118° C., and a density of 952kg/m314. The HDPE contains 2.5 wt % of Carbon black (CB), which wasfully precompounded by the supplier. For simplicity, from here onwardthe material is referred to as “PEc” (HDPE with CB). In order toinvestigate the effect of this amount of CB, a sample of the same neatHDPE resin without CB was also obtained from the same supplier, and thismaterial is referred to as “PEn” (HDPE neat). The neat HDPE has an MFIof 0.4 g/10 min, a Vicat softening temperature of 122° C., and a densityof 942 kg/m321. The PEn was used as a reference to investigate theeffect of the CB added on the neat HDPE, and the PEc was used as areference to study the effect of talc on the HDPE/CB/talc composites.

Commercial talc HAR T84 from Luzenac, France, was used as filler.

FIG. 2 is a table showing the different compositions and indicating theamount of CB and talc in the respective compositions. The compositionsshown in the table were prepared by compounding the PEc and the talc atdifferent ratios, in a twin-screw extruder with two side-feeders (ZSK 25WLE; Cooperion Werner & Pfleiderer, Germany). The temperature profileused was 180-220° C. from feed to die (above the melting temperature ofHDPE and well below its decomposition temperature). The PEc was fed intothe main hopper with a screw speed of 230 rpm while the talc was fedinto the side-feeder with a screw speed of 18 300 rpm. Two individuallycontrolled gravimetric K-tron feeders were used to control the feedingrate, both for the resin and the filler. The throughput was set to 13.67kg/h for the main feeder, while the throughput for the side-feeder wasset to obtain the desired sample composition. The extruded strand wascooled in a water bath and pelletized. The granules were then oven-driedfollowed by injection molding in an Engel ES 200/110HL Victory with ascrew diameter of 30 mm into tensile-testing bars according to ISOstandard 572-2/1A. These test bars were also used for the Charpy impactresistance measurement, thermal conductivity measurements, and waterabsorption testing. For these tests, the samples were cut to thedimensions according to the test standards.

FIG. 3 is a diagram showing specific density as a function of the weightfraction of talc in the polymer composition, clearly showing that thespecific density increases when the amount of talc in the polymercomposition increases. The increment of density is linearly proportionalto the talc content. Point A denotes the polymer composition having adensity of 1, which is the composition having approximately 8 wt % oftalc (at 2.5 wt % of CB). At levels of talc above 8 wt %, the polymercomposition is thus heavier than water and thus sinks in a sea, lake ofborehole.

FIG. 4 shows a table describing the influence of the talc loadings onthe tensile strength, elongation at yield, and elongation at break, aswell as the E-modulus. It can be seen that the tensile strength at yieldincreased gradually with increasing filler content. In contrast, PEc,which had 2.5% CB, showed a slight decrease in tensile strength comparedto HDPE with no filler (PEn). The increase in tensile strength onincorporation of talc was more evident at higher concentrations. Thestiffness was increased with both fillers: in the presence of CB, theE-modulus showed a slight increment—about 5% compared to HDPE withoutfiller—whereas the role of talc was more prominent. Despite the factthat the two additives CB and talc are totally different in terms ofshape and size, the tensile modulus increased with higher content ofeither type of particle. To evaluate the significance of differencesobserved between different composite formulations, the data (for the sixtalc concentrations from 5% to 35% and PEc as control were analyzed byone-way ANOVA at the 95% confidence level. AP-value for the tensilestrength at yield was less than α=0.05, we concluded that the effect wasstatistically significant.

FIG. 5 shows a graph of the tensile strength as a function of the fillercontent. It can be noted that the tensile strength at yield increasedgradually with increasing filler content. In contrast, PEc, which had2.5 wt % CB, showed a slight decrease in tensile strength compared toHDPE with no filler (PEn). The increase in tensile strength onincorporation of talc was more evident at higher concentrations. Toevaluate the significance of differences observed between differentcomposite formulations, the data (for the six talc concentrations from 5wt % to 35 wt % and PEc as control) were analyzed by one-way ANOVA atthe 95% confidence level. The P-value for the tensile strength at yieldwas less than α=0.05, we concluded that the effect was statisticallysignificant. An equivalent result, i.e. an increase in tensile strengthwith an increased filler concentration, was obtained for the tensilestrength at break, also shown in FIG. 5. The P-value of <0.05 for thetensile strength at break indicated that there were significantdifferences between the various composites. Regarding the elongation atyield, as anticipated, the PEn (pure HDPE) had the highest value of allcompounds. Generally, a decrease in elongation with an increase infiller content can always be expected due to the fact that the filleradded causes a reduction in chain mobility, giving rise to a rapidlydecreasing elongation at break. However all the compounds (except PEcwith 8% talc) showed a decrease in strain at yield. The stiffness wasincreased with both fillers: in the presence of CB, the E-modulus showeda slight increment—about 5% compared to HDPE without filler—whereas therole of talc was more prominent. Despite the fact that the two additivesCB and talc are totally different in terms of shape and size, thetensile modulus increased with higher content of either type ofparticle. This shows that improvement in the stiffness is more due tothe fact that rigid particulates restrict the mobility of the chainsegments of the macromolecule. Thus, improvement in the stiffness ofcomposites is only weakly dependent on particle size and shape. To acertain degree, the stiffness of the composites relies on the uniformdispersion of the particle in the matrix. It is usually expected thatagglomerates are formed when the particle amount is increased, leadingto decrease in modulus. But in the present case, the modulus increasedeven at the highest talc concentration. This indicates that the talcparticles are distributed uniformly but not randomly, and not asaggregates, even at the highest concentration.

FIGS. 6a and 6b shows impact resistance measured by a Charpy impacttest, as a function of filler content in a notched specimen edgewise(FIG. 6a ), and un-notched flatwise (FIG. 6b ). Since the talc particleis platy like, the compound cannot be considered as isotropic material,which is why it is relevant to investigate the impact resistance in twodifferent direction (flatwise and edgewise). The notched specimenincludes a small crack such that the specimen shall fail during testing.The Charpy impact test, is a test which determines how much energy amaterial absorbs during fracture. This absorbed energy provides ameasure of a given material's notch toughness. At first, withincorporation of 2.5 wt % of CB in the HDPE, the toughness was steeplyreduced by 34%. Then, in the presence of talc, the toughness graduallyimproved until at 8 wt % talc loading, were the highest value for impactwas reached, which was very close to the value for pure HDPE (83kJ/m218). After that, the impact resistance dropped gradually with anincrease in filler loading. Since some materials and composites are moresensitive to notches than others, it is advisable to compare the resultsfor notched and un-notched specimens. The Charpy impact test was alsodone with un-notched, flat wise direction and the results are shown inFIG. 6b . In the recent case also, the same trend can be noted that PEcshowed a moderate drop in impact resistance compared to PEn (as for thenotched, edgewise case). With increase in talc loading, the impactstrength increased. Statistical analysis using one-way ANOVA also gave asignificant effectiveness of talc on the impact strength compared to thePEc for both impact directions. Further the ANOVA analysis according toHsu's MCB method (multiple comparisons with the best) showed that thecomposite with 8 wt % talc was the best of all with 95% confidenceinterval.

FIG. 7 shows a graph of the impact resistance (Charpy test) and tensileelongation as a function of filler content. As can be seen, the tensileelongation and impact resistance correlate. At first, with incorporationof 2.5 wt % of CB in the HDPE, the toughness was steeply reduced by 34%.Then, in the presence of talc, the toughness gradually improved until at8 wt % loading the highest value for impact resistance was reached,which was very close to the value for pure HDPE (83 kJ/m218). Afterthat, the impact resistance dropped gradually with an increase in fillerloading.

FIG. 8 shows thermal conductivity and thermal diffusivity (thermalconductivity divided by density and specific heat capacity at constantpressure) as a function of filler content. It was found that the thermalconductivity and the thermal diffusivity increased gradually. Theenhancement of the thermal conductivity indicates that a percolatedparticle network was not formed, as we could not achieve the thermalconductivity values of pure talc. At higher filler concentrations, onewould expect that the fillers would form thermally conductive percolatednetworks (instead of isolated thermally conductive particles surroundedby the matrix), and heat can therefore flow through these channels. Themaximum thermal conductivity was up to 70% higher than for unfilled PEcat a talc concentration of 35 wt %. The talc particles were welldispersed throughout the matrix, and the particles could not form apercolated conductive path. So these results show that the heat transferoccurred according to the dispersion mechanism, with no percolation.

Other studies show that the thermal conductivity of the particle used asfiller is not always as relevant. If we compare the addition of copperparticles it has been shown that although copper (Cu) has a thermalconductivity that is about 20 times higher than for talc, the thermalconductivity of the a polymer composition comprising talc have higherthermal conductivity than the corresponding fraction of copperparticles. The interconnectivity of the filler and matrix is thereby oflarge importance for the thermal conductivity of the compounded polymercomposition.

FIG. 9 is a graph showing volumetric and specific heat capacity (Cp).What can be seen in FIG. 9 is that both volumetric and specific heatcapacity decreased with the talc increment. From the application pointof view, this means that improving heat transfer in the melt byintroducing talc particles leads to a faster production rate, whichwould be important in terms of production output and cycle time.

FIG. 10a is a graph showing thermal resistance in a geothermal pipecollector as a function of the thermal conductivity of the polymercomposition of which the pipe collector is manufactured. The thermalresistance of the pipe collector is dependent on the physical propertiesof the pipe, such as diameter, wall thickness and the occurrence ofpatterns increasing the heat exchange.

FIG. 10b shows the thermal resistance as a function of thermalconductivity for four pipe collectors having different diameter and wallthickness. From this graph, it is clear that the thermal resistance ofthe geothermal pipe collector decreases as the wall of the geothermalpipe collector is made thinner (as also shown in FIG. 10a ) and thethermal conductivity of the material increases. The result of which isthat by increasing the mechanical properties of the polymer compositionand increasing the thermal conductivity of the pipe, the thermalresistance of the pipe collector could be substantially reduced, whichmakes it possible to get the same heat exchange from the ground using ashorter pipe collector.

FIG. 11 is a table showing the result of Thermogravimetric analysisshowing the CB is an effective additive for increasing thermalstability. A decomposition of PEn of 5% occurred at 395° C. while 5%degradation in weight for PEc occurred at 435° C. Additionally, themaximum mass loss temperatures were 448° C. and 462° C., respectively.One explanation for the higher thermal stability for PEc might be themoderate enhancement of the thermal conductivity and the uniform heatdissipation. Apart from that, CB has a non-polar surface character,which is more compatible in a matrix like HDPE, as it is also non-polar.Thus, the interfacial heat transfer could be improved, reducing localoverheating and hot spots, which can delay the thermal degradation. Thetable of FIG. 11 also shows that almost 100% of the polyethylene in allsamples degraded at 550° C., and that the residue contained CB and talc.At 600° C., the CB became oxidized, which left the talc as a residue.The amount of CB can therefore be calculated by subtracting the weightloss at 650° C. from that at 3 600° C. The amount of final residueincreased correspondingly with the proportion of filler. The calculatedpercentage of residue for CB and talc for each composite was inaccordance with the values given in the table of FIG. 2.

FIG. 12 shows a graph of thermal degradation (weight as a function oftemperature) for PEn, PEc and PE with different amounts of talc. As canbe seen, PEn started to degrade at a temperature of 395° C., anddecomposition of almost 100% occurred at 570° C. As can be seen from thegraph, at high loadings, the composition comprising talc has lowerthermal stability than the PEc. The mechanism of accelerated degradationcan be explained in two ways. Firstly, the ability of the talc particlesurface to absorb stabilizers can result in reduced long-term thermalstability. Therefore, as the specific surface area of the filler isincreased, this adverse effect can be more pronounced.

Apart from increasing the thermal stability, CB increases lightstability and protection against UV. However, it has been shown that theUV-degradation of polymer compositions comprising talc is accelerated,meaning that compositions with high loadings of talc are more sensitivethe natural environment. Therefore, when choosing the talc loadedpolymer composition for a geothermal pipe collector, the increase inthermal conductivity must be weighed against the problems withaccelerated degradation of the material.

FIG. 13 shows a graph of the particle size distributions with respect tothe cumulative volume and the volume in each size fraction (particlediameter as a function of volume). The mean particle size was11.14+/−0.02 μm.

FIG. 14a shows a geothermal pipe collector 12 according to oneembodiment in which the inner surface 14 of the pipe collector 12comprises recesses or protrusions 16 extending helically in relation tothe length axis (L in FIG. 14b ) of the pipe collector 12, such that aturbulent flow is created in the direction of the length axis of thepipe collector. The recesses or protrusions 16, may be continuous ordiscontinuous in the longitudinal direction of the single pipe collector12. Usual dimensions for geothermal pipe collectors 12 are within therange 25-63 mm in diameter. The height of the indentations and/orelevations 16, which could be grooves 18 or the grooving, can be varied,but can typically be within the range of 0.2-5 mm depending on the sizeof the pipes and the wall thickness, and preferably within the range0.2-2 20 mm, for the most usual dimensions of the collector pipes 12.The grooves 18 are evenly spread around the inner circumferentialsurface of the pipe, as seen in the cross section of FIG. 14a . Thecreations of grooves on the inner surface of the pipe collector makesthe wall of the pipe collector thinner at some portions, which mayaffect some of the mechanical properties of the pipe collector 12. Thus,when removing material from the wall of the pipe collector, it may benecessary or advantageous to increase the mechanical properties of thematerial of which the pipe collector is made.

14 b shows one embodiment of the geothermal pipe collector 2 in alongitudinal section. The geothermal pipe collector comprises helicallyextending recesses or protrusions 16 which alter direction at least atsome portion along the length axis L of the pipe collector 2. Thedirection of the helical shape of the recesses or protrusions 16 can bealtered suitably at least every second meter or every meter, in thelongitudinal direction L of the pipe.

Examples of how a turbulent flow can increase the heat exchange in ageothermal pipe collector can be found in for example EP 2195586 to M.Ojala et al.

Please note that any of the polymer compositions or any combinations ofadditives mentioned herein could be used with any type of geothermalpipe collector without departing from the basic idea of the invention.

1. A geothermal pipe collector made from a polymer compositioncomprising: more than 50 wt % polyethylene, 0.1 wt %-35 wt % talc and0.5 wt %-10 wt % carbon black.
 2. The geothermal pipe collectoraccording to claim 1, wherein the polymer composition comprises 0.1 wt%-3 wt % talc.
 3. The geothermal pipe collector according to claim 1,wherein the polymer composition comprises 8 wt %-35 wt % talc.
 4. Thegeothermal pipe collector according to claim 1, wherein the polymercomposition comprises 8 wt %-15 wt % talc.
 5. The geothermal pipecollector according to claim 1, wherein the polymer compositioncomprises 8 wt %-12 wt % talc.
 6. The geothermal pipe collectoraccording to claim 1, wherein the polymer composition comprises 0.5 wt%-5 wt % carbon black.
 7. The geothermal pipe collector according toclaim 1, wherein the polymer composition comprises 1.5 wt %-3 wt %carbon black.
 8. The geothermal pipe collector according to claim 1,wherein the polymer composition comprises more than 75% polyethylene. 9.The geothermal pipe collector according to claim 1, wherein the talc istalc in which the average aspect ratio is above 1.2.
 10. The geothermalpipe collector according to claim 1, wherein the inner surface of thepipe collector comprises recesses or protrusions for increasing theturbulence of a medium flowing in the pipe collector.
 11. The geothermalpipe collector according to claim 10, wherein the recesses orprotrusions extends helically on the inner surface of the pipecollector, in relation to the length axis of the pipe collector, suchthat a turbulent flow is created in the direction of the length axis ofthe pipe collector.
 12. The geothermal pipe collector according to claim11, wherein the helically extending recesses or protrusions alterdirection at least at some portion along the length axis of the pipecollector.
 13. The geothermal pipe collector according to claim 10,wherein the recesses or protrusions extend continuously on the innersurface of the pipe collector.
 14. A method, comprising: using thepolymer composition, defined in claim 1, for increasing thermalconductivity of a geothermal pipe collector.
 15. A method, comprising:using the polymer composition, defined in claim 1, for increasingmechanical properties of a geothermal pipe collector.